1. The problem statement, all variables and given/known data It's a simple problem. What is the total mass of an oxygen atom? 2. Relevant equations Mass of electrons, protons, and neutrons, as well as their respective amount in an O atom 3. The attempt at a solution Knowing the number of e, p, and n in an oxygen atom, I can calculate the total mass of O easily. But I am just unsure if there is a more accurate way of doing this. My doubt is based on that I remember there is this mass defect thing in the nucleus which makes the total mass of a nucleus being not exactly equal to the sum of the masses of protons and neutrons it is composed of.